Process for protecting magnesium and its alloys against corrosion



Patented May 29 1934 UN TED STATES PATIENT OFFICE PROCESS FOR PROTECTING MAGNESIUM A D ITS ALLOYS AGAINST ooRRoSIoN Guy Dunstan Bengough and Lawrence Whitby,

Teddington, England, assignors to Imperial Trust for the Encouragement oi Scientific and Industrial Research, London, England No Drawing. Application April 23, 1932, Serial NO. 607,217. In Great Britain May 19, 1931 2 Claims. (01. 148-6) Our invention relates to the protection of magnesium and its alloys against corrosion and has for its object the better use of selenium and its compounds than has heretofore been achieved for 5 this purpose. In accordance with our invention we use an aqueous solution of selenious acid or of a soluble selenite to treat the magnesium or its alloys and obtain coatings containing selenium in its elemental and/ or combined forms. Selenious acid solution gives the best results, but a solution of a soluble selenite, generally sodium selenite containing selenious acid or a salt or acid known to corrode magnesium, particularly sodium chloride, gives excellent protection.

The following examples illustrate the invention:-

Example 1.-The metal to be protected is immersed in an aqueous solution of selenious acid per cent. strength at room temperature. The time of immersion is from 5 seconds to 10 minutes according to the composition of the alloy; when the alloy known as electron alloy AZM (containing in.100 parts, Mg 925, Al 6.25, Zn 0.95, and Mn 0.30) is under treatment, five minutes is suitable. A small addition of sodium chloride to the bath, such as 0.1-0.5 per cent., may improve the coating.

Insteadof immersing the metal in the solution, the latter may be applied to the metal by a painting or spraying operation, for instance by rubbing the metal with cotton-wool soaked with the solution.

Example 2.--To produce a comparatively thick and. porous coating suitable in cases where a coat of paint is subsequently to be applied, the metal is immersed in a bath of acidified aqueous sodium selenite solution, for instance a solution of sodium selenite of 6 per cent. strength to which has been added 6 per cent. by volume 0! syrupy phosphoric acid. This bath may be heated to 50 0.; the time of immersion is 1-5 minutes.

Example 3.The metal is immersed for 3 hours in an 8 per cent. aqueous solution of sodium selenite, containing about 3.2 per cent. selenious u acid and 0.1 per cent. sodium chloride and heated to about C.- C.

The protective action of the selenium coating may be improved if the coated metal is heated to about 170-225 0., for example in a mufile 65 furnace.

Special advantages of the invention are (1) that the film is self-healing to a limited extent in the presence of water, that is to say, if the film becomes slightly damaged it will re-form, at all 7 events partially, so that the metal remains protected to some extent, and (2) that the dimensional change of the metal during the process described in Example 1 is negligibly small.

The operation is rapid and simple.

The film forms an excellent basis for the application of paints and greatly increases the protection they afiord.

Having thus fully described the nature of the said invention and the best means we know of 80 carrying the same into. practical effect, what we claim as our invention is:-

1. A process of protecting magnesium and its alloys against corrosion which consists in treating the metal with an aqueous solution of selenious acid.

2. A process of protecting magnesium and its alloys against corrosion which consists in treating the metal with an aqueous solution of soluble selenite containing a material selected from the group consisting of phosphoric acid and selenious acid.

GUY DUNS'I'AN BENGOUGH. LAWRENCE WHI'ITBY. 

